KofanovV. A. “Bojanov – Naidenov Problem for the Differentiable Functions on the Real line and the Inequalities of Various Metrics”. Ukrains’kyi Matematychnyi Zhurnal 71, no. 6 (June 25, 2019): 786-800. Accessed December 2, 2020. http://umj.imath.kiev.ua/index.php/umj/article/view/1476.